1. Field of the Invention
The present invention relates to a laser tracking interferometric length measuring instrument that uses a laser beam to track the traveling distance of retroreflectors traveling in X, Y, Z three-dimensional space while performing trilateration, and to a method of measuring length and coordinates using the length measuring instrument.
2. Description of the Prior Art
There are known measuring instruments using optical interferometry. These instruments were developed for triangulation applications, and incorporate an optical interferometry system in the lens-barrel of the instrument's telescope to measure the traveling distance of mirrors. This technology is used in construction engineering works and other areas of industry that employ the triangulation method, with laser interferometry being used for the measurements that were formerly performed using a tape measure, because optical interferometry provides a higher measurement accuracy than tape measures.
Measurement objects include large-scale coordinate measuring machines, industrial robots, aircraft and other large structures, and general triangulation objects. In recent years, coordinate measuring machines have made dramatic improvements in accuracy, with some systems being capable of measuring length to an accuracy in the order of 1 μm per meter In endeavoring to construct this type of accuracy in a coordinate measuring machine, it is said to be desirable for the gage used to have an accuracy that is one-fifth to one-tenth that of the instrument being calibrated. Therefore, if the accuracy of a coordinate measuring machine is 1 μm, a gage with an accuracy that is one-fifth to one-tenth μm is desirable. However, with the existing level of technology, there is no gage having that kind of precision. Moreover, large-sized coordinate measuring machines have appeared that have a measurement volume capability of 10 meters cubed, within which the measurement accuracy is in the order of a few tens of micrometers.
As described above, it is generally desirable for a calibration gage to be calibrated to an accuracy of one-fifth to one-tenth the accuracy of the machine being calibrated. Interferometric measurement using a frequency-stabilized He—Ne laser is a method suited to measurement of such a calibration gage, but it entails many problems that still need to be resolved.
With respect to the accuracy of single-axis measurement of a coordinate measuring machine along the X axis, Y axis and Z axis, interferometric measurement using the He—Ne laser is possible, but the coordinate measurement resulting from the X, Y, Z travel gives rise to an anti-Abbe's error from the perpendicularity between the three axes and the yawing and pitching. Measuring all error takes far too long to be practical.
The prior art includes laser tracking interferometric length measuring instruments for aiming a laser beam at a moving object. FIG. 8 shows an example of such an instrument (JP-A-HEI 7-332922 and 7-332923). In this laser interferometric length measuring instrument, a mirror 610 can be rotated around the X axis and the Y axis by rotators 614 and 616, so the laser beam can be projected onto a retroreflector (not shown) attached onto a moving object. That is, the rotator 614 that supports the mirror 610 is rotatably supported by bearings 612, thereby allowing the rotator 614 to rotate freely around the X axis, relative to the rotator 616, while the rotator 616 is rotatably supported by bearings 620, enabling the rotator 616 to rotate freely around the Y axis, relative to a base plate 618.
The laser beam emitted by a laser source (not shown) is split by a polarizing cube beamsplitter 622 affixed to the base plate 618, with one of the split beams falling incident on a retroreflector 624 such as a corner cube prism or cat's eye, whereby the beam is reflected as a reference beam, and passes via the polarizing cube beamsplitter 622 and falls incident on a detector 622. The other laser beam is reflected along the Y axis by a prism 628, and then along the X axis by prisms 630 and 632, to thereby fall incident on the mirror 610.
Thus, the laser beam reflected by the mirror 610 is rotated when the rotator 616 rotates about the Y axis, and is moved vertically when the rotator 614 rotates about the X axis. This makes it possible to direct the laser beam at a retroreflector attached onto a moving object by controlling the rotation of the rotators 614 and 616. Because of the factors mentioned above, it is preferable for origin of measurement eccentricity arising from the rotation not to exceed 1.0 μm. However, with the configuration of a prior art measuring instrument, keeping the eccentricity to not more than 1.0 μm is difficult. The reasons for this can be explained with reference to the arrangement of FIGS. 8 and 9, as follows.    1. The axis 601 of the laser beam reflected by the mirror 610 is an imaginary axis of a cylinder having a certain sectional area, so mechanical contact is not possible.    2. It is difficult to have the three axes comprised by the X-Y two-axis rotation center 602 of the gimbal mount and the laser beam axis 601 intersect with one another at one point.
As the interferometric origin of measurement, there should be no movement at the point at which the three axes intersect one another. If the attitude of the telescope is changed by error at the point of intersection of the three axes, the approximate 3-axis intersection point will move eccentrically, giving rise to a primary error in the length measurement.
An object of the present invention is to provide a laser tracking interferometric length measuring instrument and method that enable eccentricity arising from rotation of the origin of interferometric measurement to be kept to not more than 1.0 μm, even when a change in the attitude of the laser beam axis results in displacement of the origin of interferometric measurement, that is, a slight movement of the center of the reflector of an articulating optical lever.
Another object of the invention is to provide a method of measuring coordinates that can utilize the calibration of a large-sized coordinate measuring machine having a measurement volume capability of 10 meters cubed.
A further object of the invention is to provide a laser interferometric length measuring method that can utilize the calibration of a high-accuracy coordinate measuring machine capable of measuring length to an accuracy in the order of 0.1 μm per meter.